Spontaneous tarnishing of silver and silver alloy surfaces exposed to the atmosphere is known to constitute a serious problem for silverware producers. In this respect it has been shown by various market investigations that one of the main reasons for the reduction in the market for silverware articles is linked to the tarnishing of their surfaces as a result of the formation of sulphurated silver compounds.
The tarnishing of silver articles, where the term “silver articles” means both solid silver articles and articles made of base metals or alloys coated with silver layers of micrometric thickness, is a process involving only the metal surface, without resulting in deep corrosion of the metal itself. It involves progressive alteration of its reflectance spectrum, giving rise to a variation in the surface colour. Although the process does not produce irreversible damage, silver tarnishing requires methodical surface cleaning which, although irksome for the final user, is even more so for distributors and retail sellers.
For this reason, the protection of silver surfaces against tarnishing has been the subject of much research over the years. In particular it has already been proposed to prevent or significantly reduce tarnishing by protecting for example the silver surface by thin polymer layers or by using waxes or surfactants which are deposited on the article surface on termination of its production. These known methods are based on the principle of preventing or generally hindering the adsorption of oxygen, of sulphurated volatile oxidants such as SO3, or of non-oxidizing sulphurated volatile compounds such as H2S, onto the metal surface.
Methods are also known which, both in the case of solid articles and of articles made of base metal alloys coated with the noble metal, prevent surface tarnishing by replacing the silver with alloys of noble metals which are less electropositive and hence less sensitive to tarnishing processes.
However all these known methods present the substantial drawback of perceptibly modifying the article appearance as they necessarily involve a variation in the material reflectance spectrum.
Methods have also been proposed for depositing spontaneously ordered molecular layers of organic molecules (known as spontaneously assembled [SAM] layers) on metals such as gold, silver and copper. These have opened new facilities for the development of processes able to reconcile the requirement of preventing or delaying surface tarnishing with the need not to appreciably alter colour and brightness.
SAM layers are monolayers or sub-monolayers of ordered organic molecules formed by molecule chemisorption on the surface of solids. In the case of surfaces of metals of group 11 (IB) of the periodic table (Cu, Ag, Au), processes are known [J. C. Love, L. A. Estroff, J. K. Kriebel, R. G. Nuzzo and G. M. Whitesides, “Self-assembled monolayers of thiolates on metals as a form of nanotechnology” Chemical Review, 105 (2005) 1103-1169] for forming said SAM layers starting from thiols in solution via the formation of direct metal-sulphur bonds starting from a suitable solution. The spontaneous formation of self-assembled layers is of interest according to the present invention, given that these layers are able to cover surfaces of arbitrary shape and size and can be obtained without recourse to ultra-high vacuum techniques, with consequent production cost reduction.
Moreover it is known that SAM layers with coating grades close to unity constitute an effective diffusive barrier to sulphurated volatile oxidant gases such as SO3 and/or non-oxidizing sulphurated volatile compounds such as H2S, towards the silver surface—and hence their relative adsorption on the same surface. In particular, it has been proposed (PCT/US1999/006775) to deposit thiols of general formula CH3(CH2)nSH on silver surfaces using procedures consisting of immersing said surfaces in suitable aqueous or organic thiol solutions. In particular, it has been noted that this procedure is effective in retarding silver oxysulphide growth and hence in partially preventing tarnishing of silver articles.
However this known process is not ideal in preventing tarnishing of articles subject to ordinary handling or even only to moderate functional use, if their surface is exposed to minimal abrasion (for example dust removal by soft cloths) or if their surfaces are brought into contact with hot liquids. The degraded protection capacity of the surface of these articles is probably due to two factors:
the assembly procedure described in WO1999048682 assumes that the formation of the prescribed covalent bonds between the surface silver atoms and the thiol sulphur atoms occurs spontaneously according to the reactionAg+RSH→Ag—SR+½H2 on the non-oxidized surface (where R is a generic alkyl); i.e.Ag—O1/2+RSH→Ag—SR+½H2Oon the oxidized surface; i.e.Ag—OH+RSH→Ag—SR+H2Oon the hydroxylated surface. However these reactions do not come to completion at ambient temperature, and consequently parts of the surface may not be coated, that is coated in a faint way from physisorbed thiols, making triggering of local oxidation processes (pitting) possible; moreover a fraction of the thiols present on the surface could be physisorbed and not chemisorbed, and hence be easily removed from the surface by mechanical or chemical means even under mild treatment conditions;
on surfaces of articles made of solid silver or of base metal or metal alloys coated with silver layers, the presence of micrometric width scoring means that the metal surface is not entirely wetted by the thiol solution and hence does not react completely with the thiols. This makes triggering of oxidative processes possible in the unprotected surface regions, and can consequently explain the appearance of surface tarnishing (typically not uniform).